1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, an encoding apparatus and an encoding method, and a computer program for performing data transmission with increased transmission capacity by a spatial multiplexing (MIMO) scheme using a pair of a transmitter and a receiver each having a plurality of antennas. In particular, the present invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, an encoding apparatus and an encoding method, and a computer program for performing delivery of a forward channel matrix, which is necessary for the transmitter to perform appropriate beamforming, from the receiver to the transmitter.
2. Description of the Related Art
Wireless networks are attracting attention as systems that free users from cable wiring in traditional wire communication schemes. Examples of common standards related to wireless networks include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15. For example, in IEEE 802.11a/g, an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme which is one of multicarrier schemes is adopted as a common standard for wireless LANs.
IEEE 802.11a/g standard supports a modulation scheme that achieves a communication speed up to 54 Mbps. However, a next-generation wireless LAN standard that can realize an even higher bit rate is being desired. As a technique for achieving faster wireless communication, MIMO (Multi-Input Multi-Output) communication is attracting attention. In IEEE 802.11n, which is an extended standard of IEEE 802.11, an OFDM_MIMO communication scheme is adopted.
MIMO achieves high-quality communications by performing beamforming between a transmitter (Beamformer) and a receiver (Beamformee) each including a plurality of antenna elements. The term beamforming as used herein refers to a method of changing the directivity of antennas by assigning weights digitally to individual transmit antennas so as to allow high-quality reception on the receiver side. Transmit antenna weights can be obtained by analysis of a forward channel matrix H from the transmitter to the receiver.
The term channel matrix H as used herein is a numeric matrix having channel information corresponding to transmit/receive antenna pairs as its elements. The channel information is a transfer function having phase and amplitude as its components. In addition, it is common to obtain a transmit beamforming matrix V and a receive weight matrix UH through singular value decomposition (SVD) of the channel matrix H into UDVH (that is, H=UDVH) (where the small H denotes conjugate transpose).
At this time, to realize beamforming in MIMO communication, a forward channel matrix HF from the transmitter to the receiver is necessary. The forward channel matrix HF can be obtained by setting up a transfer function that allows each receive antenna branch to be obtained on the receiver side, upon transmitting a known training series for exciting a channel from each transmit antenna at the receiver.
If the forward channel from the transmitter to the receiver and the backward channel are reversible, the transmitter can obtain a channel matrix H necessary for beamforming, by performing the exchanging procedure of the known training series mentioned above in the backward direction from the receiver to the transmitter. However, since an imbalance in transfer function, that is, phase and amplitude generally exists for each transmit/receive antenna branch, a forward channel matrix HF and a backward channel matrix HB do not match. Since a combination of the transfer function of the space and the transfer function of an analog portion within the apparatus is recognized as a channel by a digital processing unit on the receiver side, presence of an imbalance in analog portion of each branch leads to erroneous channel detection. Thus, even if the exchanging procedure of the known training series mentioned above is performed in the backward direction from the receiver to the transmitter, only a backward channel matrix HB can be obtained.
A method for allowing the transmitter to perform appropriate beamforming in MIMO communication is to feedback the forward channel matrix HF obtained on the receiver side to the transmitter. For example, there has been proposed a method of feeding back information related to a forward channel matrix from the receiver to the transmitter in cases where the transmitter has a smaller number of antennas than the receiver (see, for example, Japanese Unexamined Patent Application Publication No. 2007-318728).
Another method for allowing the transmitter to perform appropriate beamforming is to apply antenna calibration for correcting an imbalance to the phase and amplitude of each transmit/receive antenna branch on the transmitter side.
For example, there has been proposed a wireless communication system which obtains calibration coefficients for the respective antennas of the transmitter and receiver, and performs calibration by using the calibration coefficients for the receiver antennas and the transmitter antennas when feeding back a reference signal and also when finding a transfer function on the basis of the reference signal, thereby correcting a mismatch between forward and backward channel information matrices (see, for example, Japanese Unexamined Patent Application Publication No. 2005-160030).
The latter antenna calibration corresponds to a process of making the ratio between a transmit analog transfer function and a receive analog transfer function for each antenna branch constant. To accomplish this condition, after all, both a forward channel matrix HF and a backward channel matrix HB are necessary.
In other words, no matter which of the above-mentioned methods is used, in order to allow the transmitter to perform appropriate beamforming, it is necessary to obtain on the transmitter side the forward channel matrix HF found on the receiver side.
The number of elements of a channel matrix is determined by the combination of the number of antennas at the transmitter and the receiver. In addition, in the case of a MIMO communication system using the OFDM modulation scheme like IEEE 802.11n, channel matrices are necessary for 114 sub-carriers. In addition, considering that there are situations where beamforming is performed between a transmitter and a receiver that are manufactured by different manufactures, it is thought to be necessary to establish common rules for delivering a channel matrix used for beamforming from the receiver to the transmitter.